Rice can be the base
A scoop of rice makes a plate feel familiar in a lot of homes. Add a soft vegetable, beans, egg, or a little meat if that fits the diet.
The rice can be the part they eat
Keep the add-on small when it is new. The rice can be the part they actually eat.
Cook extra. Tomorrow’s breakfast or snack can be the same rice, and dinner did not require a new idea.
What may go on the rice
Egg only if egg is part of the diet and not an allergy. Meat only if the diet includes meat. Cheese or yogurt only if milk is fine.
A soft vegetable or beans can go on the rice for almost any of those diets. That is the flexible part.
Wheat is not required
If wheat is an allergy, rice is one of the foods that can carry the meal while bread, pasta, and oats stay off.
Keep the portion small to start. Offer more rice if they want it.
A sentence you can say
“Rice, and a little of this.”